Light source applications are well known in the art, and include those that are configured so that light is emitted relatively uniformly over a given area in a given, desired direction. The degree of uniformity and the degree of aiming are dictated by the specific application, but the emitting area is generally comparable to the device that is being illuminated.
Common applications for lighting include backlights for displays and signs as well as vehicular lights. A liquid crystal display (LCD) is commonly used in laptop computers, monitors and televisions. Because a liquid crystal produces no light of its own, but simply modulates light, it is common practice to provide lighting, called a backlight, behind the LCD. This backlight is roughly the same size as the LCD and provides a beam that is directed through the LCD toward the viewer. One type of backlight commonly comprises at least one fluorescent lamp illuminating the edges of a plastic light guide. Light is extracted from the light guide via light extraction features on the surface of the light guide (e.g., bumps, pits, and paint dots).
Illuminated signs, of the type that comprise an internal light source and a translucent outer cover with text and/or graphics formed on it, are another lighting applications. One common internal light source for this application is a row of fluorescent bulbs, with the uniformity requirements being met by placing diffuser plates between the bulbs and the outer cover.
Vehicular lights (e.g., headlights and taillights) are also examples of lighting. For example, SAE J586 July2007, Section 6.4.2, published July, 2007, calls out a minimum lighted area of 50 cm2 for brake lights, and gives details on how this is to be interpreted. In addition, FIGS. 3 to 5 and the associated text in Section 5.1.5 specify the minimum and maximum intensity that needs to be emitted in certain directions.
Several types of suitable light sources are available, and include incandescent bulbs, fluorescent tubes, discharge lamps and light emitting diodes (LEDs). Recent developments in LED technology have made them among the most efficient.
A limitation common to all of the above applications is that they are to some extent limited to flat displays. Automotive lights appear to circumvent this limitation by having a curved outer surface, but they are still limited in the sense that the light is still strongly directed irrespective of the curve. For example, typical taillights comprise an incandescent bulb in a parabolic reflector. This reflector directs the light through the outer cover of the lens with minimal deviation; only scattering due to rough surfaces causes a small amount of light to be distributed over the area of the taillight. More conspicuous is the flatness of signs and LCDs. Both of these could, in some instances, benefit from curvature but they are limited by the available types of lights to substantially flat forms.
In another aspect, it is desirable to have different light sources (e.g., different colors) for various effects (e.g., brake, back up, and running lights in an automobile tail light) in a single optical cavity.